1. Field of the Invention
The present invention is relates in general to a protection device for a power source, and more specifically to the use of a switching circuit, an overload protection circuit, and a short protection circuit for protecting an electronic device.
2. Description of the Related Art
The simplest current limiting component is a fuse, and it can be used alone or with a protection component. The fuse is capable of preventing over current impact, hence it is essential to a system as it is the final defense against system breakdown.
Presently, there are two electron protection components for power supplies in common use. One is a general fuse, which has the drawback of only one time use, and requiring replacement change after burning out. Thus, it used a chance exists that must be returned to the manufacturer for maintenance.
Another type protection component is a poly switch, which possesses the advantage of continuous use after providing the protection function, with the defect of high operational temperature. FIG. 1 is a conventional temperature and current diagram of poly switch. A represents the characteristic curve of model nanoSMD, microSMD or miniSMD, B represents the characteristic curve of the model SMDH160. The protection current of poly switch increases with a temperature in the surrounding area causing the working range of voltage to be limited by the temperature in the FIG. 1. A poly switch, however, can be used continuously and only requires periodic replacement, thus it offers an advantage over the standard fuse.
The description of a poly switch is as follows. The poly switch can protect against circuit damage by adjusting itself resistance when high current occurs. Once the current goes back to predetermined level, the poly switch automatically reverts to the normal level of resistivity. These operational characteristics of the poly switch make it rational the ideal choice for power supply and communication applications, due to the fact that changing a battery or connecting device with USB plug may generate instant high current. The poly switch typically replaces traditional glass fuse in some circuits, but is used for mainly in USB applications.
The general fuse stops current by melting and the poly switch limits current by adjusting its impedance according to current passing. Both must be triggered by over current heat to implement the protection function in the induction circuit.
The general fuse and poly switch solutions are in widespread use, however, there is limitation above. Therefore, the present invention provides better temperature characteristics than a poly switch for limiting the current and stopping the output of a power source, and without replacement every time over current occurs.